U.S. Pat. No. 4,420,754 discloses a system for measuring the relative movement between two elements, such as the scale and slide of a hand-held measuring instrument. The system includes the provision of a number of groups of supply electrodes on the slide, each of the electrodes in each group being supplied from a respective one of a multiple number of output signals from a signal generator so that all of the supply electrodes are furnished with voltages according to a cyclic pattern, the slide also having at least one receiving electrode which feeds a signal processing unit. The scale is provided with an electronic pattern comprising internally galvanically connected parts, one being a detecting part, located close to the area where the supply electrodes of the slide are moved, the other of the two parts being a transferring part which is located close to the area where the receiving electrode of the slide is moved. The movement of the slide along the scale generates a signal from the receiving electrode which is derived from the signals from at least two adjacent supply electrodes and the position of the slide is determined by a signal processing unit which identifies the amplitude ratio of the received signals.
U.S. Pat. No. 6,329,813 discloses an inductive absolute position sensor applying at least one magnetic field generator that generates a first changing magnetic flux in a first flux region. A plurality of coupling loops have a first plurality of coupling loop portions spaced at an interval related to a first wavelength along a measuring axis and a second plurality of coupling loop portions spaced at an interval related to a second wavelength along a measuring axis. One of the first plurality of coupling loop portions and the second plurality of coupling loop portions are inductively coupled to a first changing magnetic flux from a transmitter winding in a first flux region to generate a second changing magnetic flux outside the first flux region in the other of the first plurality of coupling loop portions and the second plurality of coupling loop portions. A magnetic flux sensor is positioned outside the first flux region and is responsive to the second changing magnetic flux to generate a position-dependent output signal. The output signal varies at the first or second wavelength of the corresponding other of the first plurality of coupling loop portions and the second plurality of coupling loop portions that generates the second changing magnetic flux.
The arrangements suggested in both U.S. Pat. No. 4,420,754 and U.S. Pat. No. 6,329,813 are adapted for determining linear translations between two objects.
US 2004/0207385 relates to a device for detecting the angular position of a rotatably mounted rotor relative to a stationary stator, wherein at least one electrode is arranged on the rotor and at least one electrode is arranged on the stator in such a way that the electrodes at least partially overlap in at least one rotational position of the rotor relative to the stator. US 2004/0207385 further relates to a method for measuring the rotational position of a rotor comprising at least one rotor electrode relative to a stator comprising at least one stator electrode, wherein the rotational position is detected using a capacitive coupling between the rotor electrode and the stator electrode.
US 2004/0207385 relates to the determining of angular positions between two objects such as between a rotatably mounted rotor and a stationary stator. It is a disadvantage of the arrangement suggested in US 2004/0207385 that the emitter and receiver electrodes are positioned in a manner where they take up an unnecessary amount of space. For compact systems the solution suggested by US 2004/0207385 is not applicable.
Furthermore, the arrangement suggested in US 2004/0207385 is not adapted for contact free and absolute position determination. As depicted in FIGS. 8 and 9 of US 2004/0207385 and the corresponding text in paragraphs 0053 and 0054 additional components such as force sensors or electrical resistors are required if absolute measurements are to be performed. Thus, it is a disadvantage of the arrangement suggested in 2004/0207385 that absolute measurements cannot be performed in a contact free mode of operation. If absolute, and not only relative measurement are to be performed, contact requiring elements, such as force sensors and electrical resistors, are required. Thus, it is a disadvantage of the arrangement suggested in US 2004/0207385 that only relative position measurements between rotor and stator can be performed in a contact free mode of operation.
It is a further disadvantage of the arrangement suggested in US 2004/0207385 that the rotor is only allowed to the rotated one revolution if its position needs to be tracked. Thus, if the rotor of US 2004/0207385 is rotated more than one revolution, the signals exclusively obtainable from the electrodes no longer provides an unambiguously determination of the angular position relative to the stator. Finally, the arrangement suggested in US 2004/0207385 suffers from the significant drawback that the capacitance used for determining the angular position of the rotor relative to the stator involves determining a capacitance between at least one rotor electrode and at least one stator electrode. In order to determine this capacitance electrical access needs to be provided to the rotor which is a rotatably mounted element. Providing such electrical access to a rotatably mounted member is certainly not a simple and straight forward process.
It is an object of the present invention to provide a compact and contact free arrangement for determining the absolute angular position of a dose setting member of a medication delivery device or for determining an actual dose of medicament expelled from a medication delivery device. The amount of medicament expelled from the medication delivery device is determined from an absolute angular position of a dose setting member. By absolute angular position is meant that the absolute position of the dose setting member relative to a housing of the medication delivery device is always known.